How do the LP filters in analog delays work?

[rocket]

Looking at the schematics of the Ibanez AD 80 and the Boss Dm2  I fail to understand how the antialiasing filters (around discrete transistors) work.

I have tried to simulate them (LTspice) and they didn't look like LPs either.
I must admitt though that I might have made a mistake with the simulation.

[Rob Strand]

The underlying circuit is a Sallen and Key low pass filter - there's many web references for this.   IIRC the DM2 has a third order low pass filter (with 3 caps) - this is an extension of the Sallen and Key (second order) low pass filter.

The transistor is a just a buffer.  A buffer is an amplifier with a gain of 1- for the transistor it's actually a bit less.   You will find Sallen and Key low pass filters with gain of 1 amplifiers as well as with gain.  Look at the opamp based and transistor based ckts and you will see the parallels.

If your simulation doesn't work you are doing something wrong.

[gez]

http://www.beis.de/Elektronik/Filter/Act3PoleLP.html

http://www.vzavenue.net/~jpbedinger/projects/an/3polelpf/ds1-3polelp.htm